1. Field of the Invention
The present invention takes relates to a surface mount type crystal device such as a crystal unit or crystal oscillator, and more particularly, relates to a compact crystal device that can prevent the occurrence of short-circuiting between package electrodes when the device is mounted on a wiring board.
2. Description of the Related Arts
Crystal devices are electronic components that use the piezoelectric properties of quartz crystals, and such crystal devices find use in various electronic devices. Representative examples of crystal devices include a crystal unit in which a crystal blank is hermetically encapsulated in a case, and a crystal oscillator in which a crystal element and an oscillation circuit that uses this crystal element are integrated as a single unit. With the advances in the miniaturization of electronic components in recent years, smaller sizes are also being sought for crystal devices, and various types of surface mount type crystal devices are being developed.
FIGS. 1A and 1B are a sectional view and bottom plan view showing the configuration of a surface mount type crystal unit of the prior art as an example of a surface mount type crystal device.
A surface mount type crystal unit is a device in which crystal blank 2 is accommodated within container body 1 for surface mounting and then covered by metal cover 3 to hermetically seal crystal blank 2. Container body 1 is composed of, for example, laminated ceramics and has an approximately rectangular planar outer shape, i.e., an approximately flat rectangular parallelepiped outer shape that appears rectangular when mounted on a wiring board and viewed from above. A cavity is formed in the upper surface of container body 1 for accommodating crystal blank 2. A pair of crystal holding terminals 4 are provided on the inner bottom surface of the cavity, each terminal 4 being close to a position on a respective end of one side of the inner bottom surface of the cavity. As will be explained hereinbelow, crystal holding terminals 4 are used for both electrically and mechanically holding crystal blank 2 in the cavity. In addition, mounting electrodes used when mounting container body 1 on a wiring board are provided at the four corners of the outside bottom surface of container body 1, i.e., on the surface of container body 1 that faces a wiring board when mounted on the wiring board. Each mounting electrode is formed as an approximately rectangular conductive layer. Of these four mounting electrodes, one pair of mounting electrodes 5a positioned at the two ends of one diagonal line on the outer bottom surface of container body 1 are electrically connected to the pair of crystal holding terminals 4 by way of conductive paths that are formed on the laminated plane of the laminated ceramics and the end surface of container body 1. The remaining two mounting electrodes 5b are used as grounding terminals.
As shown in FIG. 2, crystal blank 2 is composed of, for example, an AT-cut quartz crystal blank of approximately rectangular shape, excitation electrodes 6a being formed on each of the two principal surfaces. Extension electrodes 6b extend from the pair of excitation electrodes 6a toward the two sides of one end of crystal blank 2. Crystal blank 2 is secured and held within the cavity of container body 1 by securing these extension electrodes 6b to respective crystal holding terminals 4 by means of, for example, a conductive adhesive 7 at the positions at which the pair of extension electrodes 6b are led out, and crystal blank 2 is thus electrically and mechanically connected to container body 1.
A thick metal film or metal ring 8 is provided on the upper surface of container body 1 to surround the cavity, and metal cover 3 is bonded to metal ring 8 by seam welding or beam welding. Metal ring 8 is electrically connected to mounting electrodes 5b that are used as grounding electrodes by way of through-holes formed in container body 1.
As shown in FIG. 3, this type of crystal unit is typically mounted on wiring board 9 by reflow soldering. Reflow soldering is carried out by applying cream solder 10 to circuit terminals 11 provided on the surface of wiring board 9, then positioning mounting electrodes 5 of the crystal unit with respect to circuit terminals 11, and then conveying these components into a high-temperature furnace to melt cream solder 10. As a result, circuit terminals 11 and mounting electrodes 5 are electrically and mechanically bonded to each other, and the crystal unit is mounted on wiring board 9.
However, with the progress of miniaturization of the surface mount type crystal unit according to the above-described configuration, the spacing between the mounting electrodes decreases, giving rise to the occurrence of electrical short circuits between the mounting electrodes after reflow soldering. FIG. 4 shows the bottom surface of a miniaturized crystal unit. When the planar outer shape of container body 1 is, for example, 2.0 (length L) mm×1.6 (width W) mm, each mounting electrode has a length “a” of 0.7 mm and a width “b” of 0.5 mm. In this case, the spacing “c” between mounting electrodes at each side of container body 1 is 0.6 mm. This spacing “c” is smaller than the length “a” of the mounting electrodes.
Flux is typically mixed with cream solder 10 that is applied to circuit terminals 11 of wiring board 9 to facilitate the flow of melted solder. When cream solder 10 is melted, the flux protrudes from the outer peripheries of circuit terminals 11 and mounting electrodes 5, and the melted solder itself also flows from circuit terminals 11 and mounting electrodes 5 along with the flux. As the spacing “c” between mounting electrodes 5 decreases, the potential increases for connection between neighboring circuit terminals 11 and between neighboring mounting electrodes 5 and the consequent occurrence of electrical short circuits.
If the flux does not flow out, the melted solder itself will not flow onto wiring board 9 composed of an insulating material such as a ceramic material or a glass fiber reinforcement epoxy resin material.
Electrical short circuits between mounting electrodes resulting from this solder can occur not only in surface mount type crystal units, but with the miniaturization of crystal devices, can also occur in surface mount type crystal devices.